1. Field of the Invention
The present invention relates to an image reading apparatus which produces a digital image signal by reading an image from an X-ray film having the image thereon.
2. Description of the Related Art
Conventional image reading apparatuses need different parameter settings, depending on the size of a sheet, to read the image formed on the sheet. The parameters to be set for imaging include the pitch of pixels, the level of gradation of the image, the range of density of the image, the data format of image data, the destination of data transfer and the like. For instance, in image reading apparatuses for reading a medical image formed on an X-ray film, the sizes of the X-ray films are typically 14 inches.times.14 inches and 14 inches.times.17 inches for chest X-ray imaging and 8 inches.times.10 inches for breast X-ray imaging. The images formed on the X-ray films are different in characteristics depending on applications; for instance, the image for the breast is higher in resolution and wider in the range of density than that for chest imaging.
When reading such images by means of an image reading apparatus, setting levels in the pitch of pixels and range of density are different. In an 8-inch.times.10-inch film used for breast imaging, image reading is preferably performed at a fine pixel pitch and 10-bit gradation or higher in a wide density range to keep imaging degradation to a minimum. In a 14-inch.times.14-inch film and a 14-inch.times.17-inch film used for chest imaging, image reading is preferably performed at a large pixel pitch and a medium density range of 8-bit gradation or so in a narrow density range to reduce the amount of data.
In such conventional image reading apparatuses, parameters such as, for example, the pixel size, the level of gradation, the range of density, the data format of image data, and the destination of data cannot be commonly set to meet films of a plurality of different sheet sizes. An operator has to modify parameter settings such as the density range and pixel pitch each time the sheet size is changed. One set of parameter settings is applicable only to sheets of the same sheet size. If films of a plurality of different sheet sizes are read, image reading is performed under the same parameter settings, namely the same pixel pitch, the same level of gradation and the same density range.